1. Field of the Invention
The present invention relates to electrostatic discharge (ESD) protection devices and relevant methods to form those devices. In particular, the present invention relates to ESD protection devices and methods suitable for a dual gate process.
2. Description of the Related Art
As products based on integrated circuitry (ICs) become more delicate, they also become more vulnerable to the impacts of external environment, especially to ESD stress occurring when one pin of an IC is grounded and anther pin of the IC contacts an electrostatically-precharged object. Therefore, input pins, output pins, and input/output pins, the power-bus pins for an IC for communicating with external systems, must all be well equipped with ESD protection devices or circuitry to meet the minimum level of ESD robustness required by commercial applications.
NMOS (Negative-type Metal On Semiconductor Field Effect Transistor), either with the gate grounded or with the gate coupled to a positive voltage during an ESD event, have commonly been used as primary ESD protection devices for ICs. It is well known that the drain contact of an NMOS must be kept a few microns apart from the gate of the NMOS. What is implied is that the drain side of an NMOS confronting ESD stress in the front line must have a distributed resistor connected in series between the channel under the gate and a drain contacts coupled to an IC pad, and the resistance of the distributed resistor must be larger than an acceptable value. If the ESD transient current starts to localize at a weak spot near the gate, it causes the entire ESD current to rush in, thereby causing local heating and eventually damaging the NMOS. On the other hand, the distributed resistor helps to raise the potential of the adjacent diffusion area, and hence induce a more uniform ESD current flow towards the whole channel.
The advanced salicide process, which forms silicide material on drain/source regions to reduce the resistance of active regions and speeds up the circuit operation rate, however, makes construction of the above-mentioned resistor more difficult and costly.
One known solution for the problems induced by the salicide process is to use the salicide block process, which blocks the formation of silicide on certain diffusion regions. However, this solution is inefficient due to the process complexity and the extra mask required.
U.S. Pat. No. 5,721,439 (hereafter referred as ""439 patent) discloses an MOS structure comprising a number of isolated islands in the drain diffusion region (as shown in FIG. 1). The ESD transient current flows around these isolated islands from the drain contacts 10, toward the drain-gate edge, thereby increasing drain resistance to improve ESD protection.
U.S. Pat. No. 5,248,892 discloses an MOS structure comprising a resistor means whose width is substantially equal to the width of the active zone, wherein the resistor means comprises a number of strips of titanium silicide overlying a resistance zone (n-well) and extending substantially parallel to each to increase drain resistance.
U.S. Pat. No. 6,046,087 discloses an ESD protection device using a second gate as silicide-blocking mask for the drain region, wherein the second gate overlies an N-well region and separates the drain of the host transistor into two portions.
An object of the present invention is to combine two different kinds of structures for different operating voltages in the same active region.
Another object of the present invention is to reduce the ESD protection trigger voltage of an ESD protection device, thereby improving its ESD protection level.
The MOS structure for ESD protection, according to the present invention, is suitable to a dual gate process, which fabricates a thick gate oxide and a thin gate oxide on the same wafer. The MOS structure comprises at least one first island and a gate. The first island has a first conductive segment and a thin gate oxide. The first conductive segment is stacked on the thin gate oxide. The gate has a gate oxide thicker than the thin gate oxide.
The gate can be a control gate with the thick gate oxide, or a gate with field oxide.
The drain and the source of the MOS structure can be each coupled to a pad, respectively.
The advantage of the MOS structure according to the present invention is that its ESD protection trigger voltage is lower by the existence of the island. It implies that the MOS structure has a quicker trigger mechanism and a better ESD protection level when compared to the case without the island.
The present invention provides a method for early trigger of an ESD device. This method is applicable to a dual gate process as well as other IC processes. A first ESD device has a first ESD trigger voltage. A second ESD device is constructed based on the structure of the first ESD device. The second ESD device is essentially of the same construction as the first ESD device, but can have a different size, dimension, etc. An island having a thin gate oxide is incorporated with or proximate to the second ESD device, such that the second ESD device has a second trigger voltage lower than the first trigger voltage.
The fabrication method for an ESD protection device with lower trigger voltage according to the present invention is provided. An active region surrounded by an isolation region (e.g. a field oxide region) on a semiconductor wafer is formed. Within the active region, a thick gate oxide in a first region and a thin gate oxide in a second region are formed. A conductive layer on the first and second gate oxides is then formed. Then the conductive layer is patterned to form a first element on the first region and a second element on the second region.
The first element can be a gate of an MOS operated in a relative-high voltage. The second element can be an island overlapping the active region. In addition, another MOS device in another active region and operated in a relative-low voltage can be fabricated on the same wafer based on the thin gate oxide.
A diode structure with reduced ESD trigger voltage is provided in this invention. The diode comprises a first region, at least one island and a second region. The first region forms a first layer of a first conductivity type. The island is located on the first layer, having a conductive segment and a thin gate oxide. The first conductive segment is stacked on the thin gate oxide. The second region is located adjacent to the first region to form a second layer of a second conductivity type. A PN junction is formed between the first layer and the second layer. The second layer has a profile determined by the island and the second region. The island at least partially overlaps the second region for reducing ESD trigger voltage of the PN junction.
An exceeding-voltage-rating structure for enhancing ESD protection is provided, applied for a dual gate process which fabricates a first gate structure for operating in a relatively-low voltage and a second gate structure for operating in a relatively-high voltage. The exceeding-voltage rating structure comprises a first region, at least one island and a second region. The first region forms a first layer of a first conductivity type. The island has the first gate structure on the first layer. The second region is located adjacent to the first region to form a second layer of a second conductivity type and a PN junction between the first layer and the second layer. The second layer has a profile determined by the island and the second region. The island at least partially overlaps the second region to reduce ESD trigger voltage of the PN junction, and the exceeding-voltage-rating structure is operated in a relatively-high voltage application during a powered-on circuit operation.